The relationship between date of birth and individual differences in

Transcription

Personality and Individual Differences 40 (2006) 1349–1362
www.elsevier.com/locate/paid
The relationship between date of birth and individual
differences in personality and general intelligence:
A large-scale study
Peter Hartmann
a,*
, Martin Reuter b, Helmuth Nyborg
a
a
b
Individual Differences Research Unit (IDRU), University of Aarhus, Department of Psychology,
Jens Chr. Skous Vej 4, DK-8000 Aarhus C, Denmark
Abteilung für Diffentielle Psychologie, Justus Liebig University of Giessen, Otto-Behagelstrasse 10F,
35394 Giessen, Germany
Received 21 March 2005; received in revised form 1 October 2005; accepted 1 November 2005
Available online 19 January 2006
Abstract
We investigated the relationship between date of birth and individual differences in personality and intelligence in two large samples. The first sample consisted of 4000+ middle-aged male subjects from the Vietnam Experience Study; personality was measured by the MMPI items converted to EPQ (scales) and a large
battery of cognitive tests were factored to derive general intelligence, g. The second sample consisted of
11,000+ young adults from the National Longitudinal Study of Youth from 1979. g was extracted from
the ten subtests of the Armed Services Vocational Aptitude Battery.
In no cases did date of birth relate to individual differences in personality or general intelligence.
A further goal was to test Eysenck’s notion of possible relationships between date of birth and the popular Sun Signs in astrology. No support could be found for such associations.
We conclude that the present large-scale study provides no evidence for the existence of relevant relationships between date of birth and individual differences in personality and general intelligence.
! 2005 Elsevier Ltd. All rights reserved.
Keywords: Astrology; Intelligence; Personality; Date of birth; Seasonal variation
*
Corresponding author. Tel.: +45 29922400.
E-mail address: [email protected] (P. Hartmann).
0191-8869/$ - see front matter ! 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.paid.2005.11.017
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P. Hartmann et al. / Personality and Individual Differences 40 (2006) 1349–1362
1. Introduction
Several studies have investigated the relationship between date of birth and individual differences in personality or general intelligence but, in general, these studies are based on small sample
sizes. Only a couple of newer studies by Chotai, Forsgren, Nilsson, and Adolfsson (2001) and by
Chotai, Lundberg, and Adolfsson (2003), operate with appropriate sample sizes (i.e. 2130 and
1145 subjects, respectively). The present study circumvents the inherent problems with small samples by combining two unusually large populations to a total of more than 15,000 subjects. To
optimise the chances of finding even remote relationships between date of birth and individual differences in personality and intelligence we further applied two different strategies. The first one
was based on the common chronological concept of time (e.g. month of birth and season of birth).
The second strategy was based on the (pseudo-scientific) concept of astrology (e.g. Sun Signs, The
Elements, and astrological gender), as discussed in the book ‘‘Astrology: Science or superstition?’’
by Eysenck and Nias (1982).
A thorough review of the scientific literature is beyond the scope of this article since the voluminous literature dates back to the forties, but it is worth noting that many relationships have
been established between date of birth and normal intelligence, personality, and also with psychiatric diseases and differences in physical attributes (Chotai et al., 2001; Joinson & Nettle, 2005).
This present study limits itself to examining associations between date of birth, personality, and
general intelligence.
1.1. Intelligence
A series of articles suggest that subjects born during spring or summer are significantly more
intelligent than subjects born during the rest of the year, though the differences are small (e.g. Bibby, Lamb, Leyden, & Wood, 1996; Eysenck, 1995; Eysenck & Nias, 1982; Pintner & Forlano,
1943). We found no post-1996 studies suggesting that date of birth is related to intelligence,
but based on the early reports we expected to find a small but significant difference in intelligence
in favour of subjects born during the spring or summer time.
1.2. Personality
A complete review of this topic is also beyond the scope of the present report so, with one
exception, we emphasise studies from the past decade.
Forlano and Ehrlich (1941) studied the relationship between date of birth and personality and
found that subjects scoring high on Extraversion (E) and low on Neuroticism (N) were more
prone to be born during the colder months of the year. Gupta (1992) used the Hindi version of
the EPI on 104 subjects (27–38 years of age) and the I6 reports that people born during the summer score higher on Neuroticism than subjects born during winter or autumn, and lower on
Impulsiveness than subjects born during spring and autumn. Moreover, subjects born during
summer and winter appeared more Venturesome than subjects born during spring or autumn.
Chichilenko and Barbarash (2001) studied 154 male 17–20 year old young adults and found
that those born during winter, in general, scored significantly higher on Neuroticism N than those
born during autumn, who again scored significantly higher than those born during summer. The
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female subjects (N = 373) born during winter or summer scored significantly higher on N than
those born during spring. With respect to Extraversion E, males and females born during autumn
scored significantly higher than those born during summer.
A series of more recent and large-scale studies (Chotai et al., 2001; Chotai, Jonasson, Hägglöf,
& Adolfsson, 2002; Chotai et al., 2003) used the TCI to measure personality, in order to discover
differences in Novelty Seeking (NS) as a function of date of birth. These studies show that younger
subjects (below the age of 25) born during winter (October–January/March) are in general significantly more likely to score high on NS, than subjects born during the rest of the year (Chotai
et al., 2002, 2003). However, for subjects above the age of 25, the picture is reversed. In general,
subjects born during winter show lower NS scores than subjects born during the rest of the year or
during spring only (February–April). (A certain overlap between some winter and spring months
could be noted only in this presentation of the literature and is due to the summarizing nature of
the present review) (Chotai et al., 2001; Chotai et al., 2003). However, the inverted relationship
was more pronounced in females. Furthermore, the three studies by Chotai and colleagues found
that older males born during winter scored significantly lower on Persistence (PS) than others
(Chotai et al., 2001); and that younger subjects born during spring scored significantly lower
on Self Directedness (SD) than others (Chotai et al., 2002); and lastly that subjects born during
winter scored significantly higher on Harm Avoidance (HA) than others (Chotai et al., 2003).
Joinson and Nettle (2005) tested 448 subjects with a measure related to NS, namely Zuckerman’s Sensation Seeking (SS) scale, and found that subjects aged 20–45 born during October–
March scored significantly higher on SS than others, while subjects aged 46–69 and born during
October–March scored lower than subjects born during the rest of the year.
All together the results suggest that there are individual differences in NS or SS or E as a function of month or season of birth, and that they vary as a function of age and gender. The present
study operates with Eysenckian personality dimensions (EPQ), and can therefore approach the
alleged relationship using alternative personality scales to test whether findings are valid across
instruments. The EPQ factors have been found to be related to the TCI and ZKPQ from the
Zuckerman and Kuhlman’s Personality Questionnaire (see Zuckerman & Cloninger, 1996). Based
on the positive correlations between Novelty Seeking, Psychoticism (P), and Extraversion (E) we
expected to find differences in E and P as a function of date of birth. Other TCI factors, which
have also been found to be associated with date of birth, are also related to the EPQ. For instance,
Persistence is negatively related to Psychoticism; Self Directedness is related negatively to Psychoticism and Neuroticism. Finally, Harm Avoidance is negatively related to Extraversion and positively to Neuroticism (Zuckerman & Cloninger, 1996). Based on the previous findings and
presuming that the instruments applied relate to Eysenck’s factors, it seems reasonable to expect
that the present study will confirm a relationship between date of birth and individual differences
in personality.
1.3. Astrology
Several reviews of the literature on astrology (Dean, Nias, & French, 1997; Eysenck & Nias,
1982) show, in general, little support for any claim of an effect of astrology. As highlighted by
Dean et al. (1997), the effect sizes of astrologer’s judgment amount to no more than 0.05, indicating very little practical relevance of astrology. When astrological effects are in fact observed, they
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Table 1
Translation of Sun Signs to Elements and gender
Sun Sign
From (dd-mm)
To (dd-mm)
Element
Gender
Aries
Taurus
Gemini
Cancer
Leo
Virgo
Libra
Scorpio
Sagittarius
Capricorn
Aquarious
Pisces
22-03
22-04
23-05
23-06
24-07
25-08
25-09
25-10
24-11
23-12
22-01
20-02
18-04
19-05
19-06
20-07
21-08
21-09
21-10
20-11
19-12
18-01
17-02
18-03
Fire
Earth
Air
Water
Fire
Earth
Air
Water
Fire
Earth
Air
Water
Masculine
Feminine
Masculine
Feminine
Masculine
Feminine
Masculine
Feminine
Masculine
Feminine
Masculine
Feminine
usually can be explained better by non-astrological factors like the Barnum effect (i.e. the ‘‘. . .tendency for people to identify with personality descriptions of a general and vague nature. . .’’) or by
acquaintance with Sun Signs and so on (Eysenck & Nias, 1982, p. 43). More recently, Clarke,
Gabriels, and Barnes (1996) conducted a study on the effect of astrology in a sample of 190 subjects, testing two hypotheses. First, subjects born with the sun, moon and/or ascendant in positive
signs (or the masculine gender) will be more extraverted than subjects born with these planets in
the negative signs (feminine gender). Second, subjects born with the three planets in water signs
will be more emotional than subjects born in non-water signs (confer with Table 1 showing which
Sun Signs correspond to which Elements and astrological gender). Several t-tests indicated that
subjects born with the sun and the moon in positive signs (masculine gender) were in fact more
extraverted than subjects born with the sun and moon in negative signs. However, the significant
p-values (about 0.03) would loose their significance had the authors used the Bonferroni correction to compensate for multiple testing.
Based on these results we did not expect to find relevant relationships between astrology (Sun
Signs, Elements and astrological gender) and individual differences in general intelligence and
personality.
2. Methodology
2.1. Subjects
We drew subjects from two sources. First, archival data from the Vietnam Experience Study
(VES) were kindly provided by Centers for Disease Control (1988, 1989). The purpose of the
VES study (to assess possible long term effect of military service for male veterans), the subjects
(N = 4462) and the test instruments used have been described in details elsewhere (Centers for
Disease Control, 1988, 1989; Nyborg & Jensen, 2000, 2001). The VES sample is fairly representative of the US male population with respect to education, income, occupation, and race. However, subjects scoring below the 10th percentile in the pre-induction cognitive aptitude test were
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excluded, in accordance with a US Congress mandate, and this obviously truncates the lower-end
tail of the ability distribution. For the present purpose we included a total of 4321 middle-aged
male subjects in the analysis of month of birth and season of birth (see below). The veterans were
inducted during 1965–71 (mean age = 19.92; SD = 1.72) and re-tested in 1985–86 (at mean
age = 38.35; SD = 2.52) with a mean testing (span = 17.90; SD = 1.86). Intelligence was measured twice—at induction and during re-testing, whereas personality was monitored only at
retesting.
Second, further data was drawn from the National Longitudinal Study of Youth 1979
(NLSY1979; NLSY1979 User’s Guide, 2004). The purpose of this study, the subjects, and the
instruments used have been described elsewhere (Evans, 1999; Legree, Pifer, & Grafton, 1996).
The sample comprised of 11,448 young adult subjects (male N = 5749, female N = 5699). Age
ranged between 15 and 24 years (mean age = 19.6; SD = 2.26).
2.2. Instruments
2.2.1. The VES study
2.2.1.1. Intelligence. Principal Component Analysis (PCA) was conducted on 19 cognitive variables (Grooved Pegboard Test, left and right hand; Paced Auditory Serial Addition Test; Rey-Osterrieth Complex Figure Drawing, direct copy, immediate recall and delayed recall; Wechsler Adult
Intelligence Scale-Revised, general information and block design; Word List Generation Test;
Wisconsin Card Sort Test; Wide Range Achievement Test; California Verbal Learning Test;
Army Classification Battery, verbal and arithmetic reasoning, administrated twice; Pattern Analysis
Test; General Information Test; Armed Forces Qualification Test).
The First Principal Component (PC1) was extracted to reflect general intelligence, and the g
factor scores were used to estimate individual ability level.
2.2.1.2. Personality. The VES database also contained clinical MMPI-II data. Following the
guidelines by Gentry, Wakefield, and Friedman (1985) we derived the four Eysenckian Personality
dimensions: Psychoticism (P), Extraversion (E), Neuroticism (N), and Social Desirability (L: Lie
scale). The reliabilities of the scales were: P (a = 0.74), E (a = 0.60) and N (a = 0.92). These values
are acceptable, but the L (a = 0.32) reliability was too low. Thus, L scale results must be interpreted with caution.
2.2.1.3. Date of birth (VES). Five different indicators of date of birth were computed based on the
birth dates of the subjects:
(a) Month of Birth: The month during which the subject was born (January, February, etc.).
(b) Season of Birth: Seasons of birth was somewhat rather arbitrary defined as: Spring (March–
May), Summer (June–August), Fall (September–November) and Winter (December–
February).
(c) Extended Winter vs. Summer: A cosinur analysis by Chotai et al. (2002, 2003) investigated
whether any systematic fluctuations could be fitted to a cosines curve (!). The results suggested systematic cosines fluctuations from ‘‘Extended Winter’’ (October–March) to
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(d)
(e)
(f)
(g)
‘‘Extended Summer’’ (April–September). Based on these findings it was decided to construct
two groups (Extended Winter/Summer) in order to investigate any seasonal differences based
on this division of the year obtained from Chotai et al. (2002, 2003) studies.
Extended Spring vs. Fall: Chotai et al. (2003) further suggest that the months January–June
(Extended Spring) could be different from July–December (Extended Fall).
Sun Sign: The astrological Sun Sign is the commonly known star sign based on the place of
the sun in relation to the date of birth of the subject. This Sun Sign is commonly associated
with different attributes for different signs (Eysenck & Nias, 1982, for details). Where one
Sun Sign stops and another starts is not completely agreed upon among astrologers, due
to slightly different methods of estimating an individual Sun Sign. This leads to minor differences in the dates used to incorporate people into certain Sun Signs, typically diverging by
1 day across methods. In order not to risk inclusion of subjects in an incorrect Sun Sign
group, it was decided to exclude subjects born on the day, the day before, and the day after
any of these division points provided by the two methods. This resulted in a gap of ca.
2–3 days around a division point from which subjects were excluded (see Table 1 for illustration). This conservative categorisation reduced the number of subjects for Sun Signs analysis
from 4321 to 4054.
Sun Sign associated with an Element (i.e. fire, water, air, and earth): Subjects assigned to a Sun
Sign group were also assigned to an Element group (see Table 1 for details).
Sun Sign associated with astrological gender (i.e. masculine/feminine): Subjects assigned to a
sun sign group were also assigned to a gender group (see Table 1 for elaborations).
2.2.2. The NLSY1979 study
2.2.2.1. Intelligence. A Principal Axis Factoring (PAF) was conducted on the 10 subtests of the
Armed Services Vocational Aptitude Battery (ASVAB): (General Science; Arithmetic Reasoning;
Word Knowledge; Paragraphs Comprehension; Numerical Operations; Coding Speed; Automobile and Shop Information; Mathematics Knowledge; Mechanical Comprehension; Electronics
Information). The first Principal Axis Factor (PAF1) was applied to derive general intelligence,
g, and the g factor scores were then used as an estimate of individual g.
2.2.2.2. Personality. We did not have access to personality data for NLSY1979 subjects.
2.2.2.3. Date of birth (NLSY1979). As NLSY1979 study only provides the month of birth of the
subjects, we were restricted to perform analyses based on month of birth, season of birth,
Extended Spring vs. Fall, and Extended Winter vs. Summer.
2.3. Statistical evaluation
For all analyses the conventional alpha value of 0.05 (two tailed) was used.
2.3.1. The VES study
The four personality factors and the PC1 scores were entered one by one as the dependent variables in an ANCOVA design, while controlling for the remaining four variables and the chronolog-
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ical age of the subjects. The categorical predictor variables were the seven indicators of date of birth,
namely month of birth (12 groups), season of birth (four groups), Extended Spring vs. Fall (two
groups), Extended Summer vs. Winter (two groups), Sun Sign (12 groups), Elements (four groups),
and astrological gender (two groups). This resulted in a total of seven times five ANCOVA’s.
2.3.2. The NLSY1979 study
Only four ANCOVA’s were performed with the PAF1 score as the dependent variable and
month of birth, season of birth, Extended Spring vs. Fall, Extended Summer vs. Winter served
as categorical predictor variables while controlling for age.
3. Results
Due to the space limitation only figures for the relationship between the dependent variable
(personality and general intelligence) and month of birth are reported, since the other results
are categorizations of these, and can accordingly to some extent be derived from these. Furthermore, all significant relationships are also reported.
3.1. The VES study
As can be seen from Figs. 1–6, out of 35 analyses only one reached the specified level of significance (see Figs. 1–6). This was for Extended Spring vs. Fall (Fig. 6), but the difference amounted
Fig. 1. Predicting general intelligence g (PC1) from month of birth.
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Analysis of variance with control for E, N, L, PC1 and Age
Month of birth effect
F(11, 4304)= 0.83; p=0.61
8.0
7.8
P score
(mean=7.15; SD=3.40)
7.6
7.4
7.2
7.0
6.8
6.6
6.4
1
2
3
4
5
6
7
8
9
10
11
12
11
12
Month
Fig. 2. Predicting Psychoticism (P) from month of birth.
F(11, 4304) = 0.96; p = 0.48
16.0
15.8
15.6
E score
(mean=14.75; SD=4.56)
15.4
15.2
15.0
14.8
14.6
14.4
14.2
14.0
13.8
1
2
3
4
5
6
7
8
9
10
Month
Fig. 3. Predicting Extraversion (E) from month of birth.
Analysis of variance with control for P, E, L, PC1 and Age
F(11, 4304) = 0.68; p = 0.76
14.0
13.8
N score
(mean=13.05; SD=5.00)
13.6
13.4
13.2
13.0
12.8
12.6
12.4
12.2
12.0
1
2
3
4
5
6
7
8
9
10
11
12
Month
Fig. 4. Predicting Neuroticism (N) from month of birth.
Analysis of variance with control for P, E, N, PC1 and Age
F(11, 4304) = 0.38; p = 0.96
8.4
8.3
L score
(mean=7.93; SD=2.40)
8.2
8.1
8.0
7.9
7.8
7.7
7.6
7.5
1
2
3
4
5
7
6
8
9
10
11
Month
Fig. 5. Predicting Social Desirability (Lie score, L) from month of birth.
12
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Fig. 6. Predicting general intelligence g (PC1) from Extended Spring vs. Fall.
to less than one IQ point in favour of subjects born in the Extended Fall. No significant relationships could be observed between date of birth and personality. A simple eye-ball inspection of the
five graphs for month of birth did not suggest any systematic relationship besides the ones already
tested.
It was not possible to conduct separate analyses for sex as the VES database did not contain
female data.
3.2. The NLSY1979 study
From the four analyses (see Figs. 7–9), the relationship between month of birth and PAF1 score
came close to significance (p " 0.08), whereas the relationship between PAF1 score and season of
birth reached significance (p < 0.025) as well as for Extended Spring vs. Fall (p < 0.002). Fig. 8
illustrates that subjects born during spring obtained higher g than subjects born during fall. Subjects born during winter or summer seem to score closer too, but less than subjects born during
spring. Fig. 8 also shows that the relationship is very modest (less than one IQ point) even if
statistically significant. These results are corroborated by the Extended Spring vs. Fall results,
where subjects born during the Extended Spring score somewhat higher g (but less than one IQ
equivalent point). A simple eye-ball inspection of the graph for month of birth did not suggest
any systematic relationship besides the ones already seen.
Conducting the analyses separately for gender showed that the ANCOVA for month of birth
and PAF1 was significant for males (p < 0.05) but not for females (p > 0.1). This pattern is also
Fig. 7. Predicting general intelligence g (PAF1) from month of birth.
Fig. 8. Predicting general intelligence g (PAF1) from season of birth.
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Fig. 9. Predicting general intelligence g (PAF1) from Extended Spring vs. Fall.
found when substituting month of birth by season of birth. The ANCOVA reached significance
(p < 0.001) for males but not for females (p > 0.1). The Extended Spring vs. Fall analysis showed
the same pattern, where the relationship was significant for males (p < 0.006) but not for females
(p < 0.1). Apparently the overall relationship between season of birth and Extended Spring vs.
Fall is mediated by gender.
4. Discussion
The large-scale analyses of the VES data show no relationships between date of birth and personality as defined by the four Eysenckian personality dimensions. Given the rather liberal level of
significance used in relation to the numerous analyses performed, it is rather surprising not to find
at least one spurious relationship. Based on the research literature we expected to find at least
some differences in personality as a function of date of birth.
The reason for the discrepancy is unknown. The small effect sizes could be a factor, but the
present samples are large enough to trace even small effects. Another reason could be the age
of our subjects. Perhaps personality differs over time, and one could then argue that our middle-aged ‘‘personality’’ sample is at a point where the differences in personality as a function of
date of birth are shifting and therefore differences will be minimal. However, there is consensus
that personality tends to be stable over life-span; but it cannot be excluded that a larger age span
or different age categories than in the present study might have provided different results. More-
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over, as argued by a reviewer, it cannot be ruled out that the selection bias inherent in this sample,
i.e. all armed service veterans, could have modified the effect of date of birth. However, multiple
socio-economic indicators demonstrated that veterans who had served in Vietnam about 20 years
later in general fared as well in society as those who did service elsewhere at the same time, and
that both groups distributed themselves socio-economically as expected for the general population
(Centers for Disease Control, 1988, 1989). Another argument was raised by a reviewer: Perhaps
years of military service and possible war experiences could affect the personality of the subjects?
Since the personality measures were collected during the re-testing phase in 1985/86 at middle-age,
one cannot rule out that an effect of date of birth has been eliminated on this account. In sum, the
selection bias of the present sample was probably lighter than that of most college samples
typically investigated, and no serious personality bias could be traced.
With respect to general intelligence we found evidence that subjects born during the Extended
Fall from June–December are slightly more intelligent (less one IQ point) than subjects born during the Extended Spring. However, these results contrast the findings in NLSY1979 database,
which showed approximately the same difference but in the opposite direction. However,
NLSY1979 findings dovetail with the research literature and are therefore more likely to be correct. The question is, however, whether the findings are solid and whether the relationship between date of birth and general intelligence may be mediated by the age of the subjects as
perhaps for personality. It is even possible both findings are artefacts of using rather liberal p-values in the light of the number of analyses performed and the huge sample size. When the p-values
are subjected to the Bonferroni correction for 39 analyses, then no significant results survive. In
any case, even if the differences are, in fact, valid, the effect size is far too low to be of any practical
relevance.
Our findings on astrology concord in general with the reviews of the research literature (Dean
et al., 1997; Eysenck & Nias, 1982). In fairness, the present study cannot falsify astrology at large
since the position of planets other than the sun might also have an effect. However, as pointed out
by Eysenck and Nias (1982, p. 31 and 49) if there is some truth to astrology then some general
effects of prominent astrological factors like Sun Signs should be detectable using large enough
samples. The present large-scale study certainly found no independent effects of Sun Signs,
Elements, or gender, and thus yields no support for the common claims of astrology.
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